Prevention of mother-to-child transmission of HTLV-1.Human T-lymphotropic virus type I (HTLV-1) is closely linked with serious diseases, such as adult T-cell leukemia (ATL), tropical spastic pararesis/HTLV-1 associated myelopathy (TSP/HAM) and HTLV-1 assosiated retinophathy (HAR). Unfortunately, it is not currently possible to care the diseases. The only way aviable to provent thease diseases is breaking the cycle of HTLV-1 endemy.HTLV-1 is heavily endemic in Nagasaki. The prevelance of carriers in Nagasaki is approximately 4% in pregnant women.The main route of HTLV-1 transmission has been established as maternal, specifically via breast milk. The incidence of maternal transmission is 20-30%. Maternal transmission were remearkably decreased to the level of 3% in formula-fed.Approximately 90% of maternal infection was blocked by refraining of breast feeding by carrier mothers.The incidence of infection in group of breast-fed over 6 month was 16.2%, under 6 month was 10.4% and in the group of formula-fed was 2.7%.Comparing the long term breast-fed group and the short term-breast-fed group, I can't find any differene between them.